Summary:
Add printing of stall time in DB Stats:

Sample outputs:

** DB Stats **
Uptime(secs): 53.2 total, 1.7 interval
Cumulative writes: 625940 writes, 625939 keys, 625940 batches, 1.0 writes per batch, 0.49 GB user ingest, stall micros: 50691070
Cumulative WAL: 625940 writes, 625939 syncs, 1.00 writes per sync, 0.49 GB written
Interval writes: 10859 writes, 10859 keys, 10859 batches, 1.0 writes per batch, 8.7 MB user ingest, stall micros: 1692319
Interval WAL: 10859 writes, 10859 syncs, 1.00 writes per sync, 0.01 MB written

Test Plan:
make all check
verify printing using db_bench

Reviewers: igor, yhchiang, rven, MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D31239